inference: improve isdefined_tfunc accuracy for Unions (#43036)

Author: aviatesk

base/compiler/tfuncs.jl

@@ -313,6 +313,12 @@ function isdefined_tfunc(@nospecialize(arg1), @nospecialize(sym))
                 end
             end
         end
+    elseif isa(a1, Union)
+        t = Bottom
+        for u in uniontypes(a1)
+            t = tmerge(t, isdefined_tfunc(u, sym))
+        end
+        return t
     end
     return Bool
 end

test/compiler/inference.jl

@@ -1153,6 +1153,11 @@ end
     @test isdefined_tfunc(NamedTuple{(:x,:y),<:Tuple{Int,Any}}, Const(:y)) === Const(true)
     @test isdefined_tfunc(NamedTuple{(:x,:y),<:Tuple{Int,Any}}, Const(:z)) === Const(false)
 end
+struct UnionIsdefinedA; x; end
+struct UnionIsdefinedB; x; end
+@test isdefined_tfunc(Union{UnionIsdefinedA,UnionIsdefinedB}, Const(:x)) === Const(true)
+@test isdefined_tfunc(Union{UnionIsdefinedA,UnionIsdefinedB}, Const(:y)) === Const(false)
+@test isdefined_tfunc(Union{UnionIsdefinedA,Nothing}, Const(:x)) === Bool
 
 @noinline map3_22347(f, t::Tuple{}) = ()
 @noinline map3_22347(f, t::Tuple) = (f(t[1]), map3_22347(f, Base.tail(t))...)